A refrigeration system sometimes needs to use multiple compressors at the same time. For example, the manifolding of compressors is being used in the air conditioning and refrigeration industry more and more frequently. Compressors connected in parallel have advantages such as convenience in energy modulation, convenience in maintenance of a single shutdown compressor, and low cost. Lubrication oil is indispensable during running of the compressors. However, due to different displacements and different piping designs between the multiple compressors, a compressor, especially a scroll compressor with a low-pressure chamber, may be damaged due to the lack of lubrication oil.
Therefore, it is necessary to manage oil levels of multiple compressors. In conventional oil level management, an active oil return apparatus is used in the refrigeration industry. However, the active oil return apparatus is not as applicable to commercial or light commercial air conditioning due to its high cost and complex system structure.
Alternatively, the oil level may also be managed by way of piping design, but this cannot effectively control the oil level of a compressor. Therefore, the conventional oil level management cannot meet requirements for both low cost and high reliability.
A conventional refrigeration system is widely used in an air conditioning device for cooling and heating indoor air and used in other refrigeration machines. A compressor group in the conventional refrigeration system includes multiple compressors. One of the multiple compressors is a “first” compressor. The first compressor may be a compressor with modulated capacity (or with variable displacement) or may be a fixed capacity compressor. To enable the refrigeration system to be operated in a partial load mode, others of the multiple compressors are “second” compressors connected in parallel. The second compressors can work intermittently according to load demands. When capacity requirement is precise, the first compressor further has a capacity adjustment (variable capacity) capability. In order to increase the precision of reaching the required capacity, the first compressor further has an ability to modulate capacity according to a request.
Specifically, in a conventional refrigeration system, there are several methods for balancing lubrication oil among the first compressor and the second compressors. To balance oil among a plurality of compressors, a method depends on an oil balancing pipe among compressors. Another method depends on an oil separator at a discharge pipe. However, none of the conventional methods can provide a reliable oil balancing solution in a partial load condition. If no oil balancing pipe is provided for a refrigeration system, a compressor having a small capacity tends to be short of oil. In a refrigeration system without an oil balancing pipe, a compressor having a larger capacity may reach an oil-starvation state faster.
Currently, an oil balancing pipe is provided in a conventional compressor group. The oil balancing pipe is connected in parallel or in series to an oil sump of a compressor. In some solutions, an additional gas balancing pipe is installed among the compressors, so as to reduce a pressure difference between compressors caused by different refrigerant flows.
In a conventional compressor group, when compressors are operated with different capacities, an oil return pipe and an oil balancing pipe for the compressors cannot solve the oil balancing problem in a partial load mode. It has been proven in practice that in some conditions (e.g., there is a large pressure difference between different compressors due to different compressor capacities), oil may be sucked from a compressor having a higher pressure and enter a compressor having a lower pressure. In addition, a gas balancing pipe may be helpful to reduce a pressure difference. However, the use of the gas balancing pipe requires changes to the structure of a compressor and requires more piping connections and welding work, resulting in a complex system.
Therefore, there is no reliable and economic oil balancing solution in the conventional art.